The present invention relates to a novel method for producing a substantially desulfurized hydrocarbon fuel stream, particularly for hydrogen generation, and more particularly for use within a fuel cell processing train, by passing a nondesulfurized hydrocarbon fuel stream, particularly natural gas, propane or liquefied petroleum gas (LPG), through a sequential sulfur adsorbent system at temperatures less than 100° C., wherein the sequential sulfur adsorbent system contains in sequence a zeolite Y sulfur adsorbent, preferably exchanged with copper ions, and at least one selective sulfur adsorbent. The present invention further relates to a process for producing hydrogen within a fuel cell processing train from a substantially desulfurized hydrocarbon fuel stream, particularly desulfurized natural gas, propane or LPG, wherein the hydrocarbon fuel stream is desulfurized using the above-described sequential sulfur adsorbent system. The present invention further includes the desulfurization system described above utilized for hydrogen generation, particularly within a fuel cell processing train, which system desulfurizes hydrocarbon fuel streams, particularly comprising natural gas, propane or LPG, at temperatures as low as ambient temperature, even if water is present in the fuel stream.
For hydrogen generation, particularly for use in a conventional low temperature fuel cell processing train, such as a proton exchange membrane (PEM) fuel cell, which is suitable for use in a stationary application or in a vehicle, such as an automobile, the hydrocarbon fuel stream can be derived from a number of conventional fuel sources with the preferred fuel sources including natural gas, propane and LPG. In a conventional hydrogen generation system, particularly a fuel cell processing train, the hydrocarbon fuel stream is passed over and/or through a desulfurization system to be desulfurized. The desulfurized hydrocarbon fuel stream for such fuel cell processing train then flows into a reformer wherein the fuel stream is converted into a hydrogen-rich fuel stream. From the reformer the fuel stream passes through one or more heat exchangers to a shift converter where the amount of CO in the fuel stream is reduced. From the shift converter the fuel stream again passes through various heat exchangers and then through a selective oxidizer or selective methanizer having one or more catalyst beds, after which the hydrogen rich fuel stream flows to the fuel cell stack where it is utilized to generate electricity.
Raw fuels, in gaseous or liquid phase, particularly natural gas, propane and LPG, are useful as a fuel source for hydrogen generation, particularly for fuel cell processing trains. Unfortunately, virtually all raw fuels of this type contain relatively high levels, up to as high as 1,000 ppm or so, but typically in the range of 1 ppm to 500 ppm, of various naturally occurring sulfur compounds, such as, but not limited to, carbonyl sulfide, hydrogen sulfide, thiophenes, such as tetra hydro thiophene, dimethyl sulfide, various mercaptans, including ethyl mercaptan and tertiary butyl mercaptan, disulfides, sulfoxides, other organic sulfides, higher molecular weight organic sulfur compounds, and combinations thereof. In addition, because hydrocarbon fuel streams, particularly natural gas, propane and LPG, may have different sources of origin, the quantity and composition of the sulfur compounds that may be present in the fuel streams can vary substantially. Further, these fuel stream sources may also contain water.
The presence of sulfur-containing compounds, in a hydrocarbon fuel stream can be very damaging to components of the fuel cell processing train, including the fuel cell stack itself, and such compounds must therefore be substantially removed. If not substantially removed, the sulfur compounds may shorten the life expectancy of the components of the fuel cell processing train.
An especially efficient desulfurization system is necessary for use in such fuel cell processing trains as they generally only contain a single desulfurization system. Further, desulfurization systems for such uses must have high capacity, as they may need to be in use for an extended period of time before replacement.
Several processes, conventionally termed “desulfurization,” have been employed for the removal of sulfur from gas and liquid fuel streams for hydrogen generation. Adsorption of sulfur-contaminated compounds from these hydrocarbon streams using a “physical” sulfur adsorbent is the most common method for removal of sulfur compounds from such hydrocarbon fuel streams because of their relatively low capital and operational costs. (For purposes of this specification, the terms “adsorption” and “absorption” as well as “adsorbents” and “absorbents” each have the same, all inclusive meaning.) While physical adsorbents are useful, they can desorb the sulfur compounds from the adsorbent under certain operating conditions. In addition, there are often limits on the quantity of sulfur compounds which can be adsorbed by such physical sulfur adsorbents.
An additional type of adsorbent that has been useful as a desulfurization agent is a “chemical” sulfur adsorbent. However, chemical desulfurization normally requires the desulfurization system to be heated to temperatures of 150° C. to 400° C. before the nondesulfurized hydrocarbon fuel streams can be effectively desulfurized by the chemical adsorbent desulfurization system. In addition, other operational problems may occur when such chemical desulfurization processes are utilized.
While many different desulfurization processes have been suggested for hydrocarbon fuel streams, there is still a need for improved processes for desulfurization to achieve enhanced adsorption of sulfur components over an extended range of sulfur concentrations, especially at relatively low operating temperatures and pressures, and for extended periods of time. In addition, these improved processes for desulfurization must be able to achieve enhanced adsorption of sulfur compounds even when water is present in the feed stream. Further, there is a need for improved desulfurization systems to adsorb substantial quantities of a wide range of sulfur compounds, including particularly dimethyl sulfide, various mercaptans, such as ethyl mercaptan and tertiary butyl mercaptan, hydrogen sulfide, carbonyl sulfide, tetra hydro thiophene, disulfides, sulfoxides, other organic sulfides, various higher molecular weight sulfur-containing compounds and combinations thereof. Further, it is important that these improved desulfurization systems absorb this broad range of sulfur compounds effectively for an extended period of time to delay “breakthrough” of sulfur compounds as long as possible. “Breakthrough” occurs when the amount of any sulfur compound remaining in the feed stream after desulfurization is above a predetermined level. Typical “breakthrough” levels for sulfur compounds occur at less than 1 ppm. Breakthrough by virtually any of the sulfur compounds present in the hydrocarbon fuel stream is disadvantageous as substantially all sulfur compounds can cause damage to components of a hydrogen generation system, particularly for a fuel cell processing train.
In addition, some prior art adsorbents, while effective as adsorbents for some sulfur compounds, can synthesize the production of sulfur compounds even as they are removing some of the naturally occurring sulfur compounds that are present in the hydrocarbon fuel stream. (These newly produced sulfur compounds are referred to herein as “synthesized sulfur compounds.”) It is important that the desulfurization system avoid the production of synthesized sulfur compounds to the greatest extent possible and for the longest period of time possible.
The foregoing description of preferred embodiments of the invention provides processes, systems and products that address some or all of the issues discussed above.